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When selecting NUMPAD+ the TrackIR software is telling me: This key/ button is already in use.

That's wrong. What's the reason for it (I used this key some time before).
In a new profile it does not happen - but I do not like to fiddle all the settings again.

Any idea/ suggestion?

It's likely that the key is still assigned to another function, or that it is somehow stuck in the profile file.

You can always create a new profile, assign + to a function such as center, then save it. Edit the new profile's .xml file with notepad, look at the number assigned to the hotkey, then compare that to the old profile to find where it is stored. If you'd like, you can just copy and paste both xml files in here and I can look at them for you.